243 research outputs found
Equilibrium Sequences and Gravitational Instability of Rotating Isothermal Rings
Nuclear rings at centers of barred galaxies exhibit strong star formation
activities. They are thought to undergo gravitational instability when
sufficiently massive. We approximate them as rigidly-rotating isothermal
objects and investigate their gravitational instability. Using a
self-consistent field method, we first construct their equilibrium sequences
specified by two parameters: alpha corresponding to the thermal energy relative
to gravitational potential energy, and R_B measuring the ellipticity or ring
thickness. Unlike in the incompressible case, not all values of R_B yield an
isothermal equilibrium, and the range of R_B for such equilibria shrinks with
decreasing alpha. The density distributions in the meridional plane are steeper
for smaller alpha, and well approximated by those of infinite cylinders for
slender rings. We also calculate the dispersion relations of nonaxisymmetric
modes in rigidly-rotating slender rings with angular frequency Omega_0 and
central density rho_max. Rings with smaller alpha are found more unstable with
a larger unstable range of the azimuthal mode number. The instability is
completely suppressed by rotation when Omega_0 exceeds the critical value. The
critical angular frequency is found to be almost constant at ~ 0.7
sqrt(G*rho_c) for alpha > 0.01 and increases rapidly for smaller alpha. We
apply our results to a sample of observed star-forming rings and confirm that
rings without a noticeable azimuthal age gradient of young star clusters are
indeed gravitationally unstable.Comment: 17 figures and 2 tables; Accepted for publication in the Ap
A Scalable Deep Neural Network Architecture for Multi-Building and Multi-Floor Indoor Localization Based on Wi-Fi Fingerprinting
One of the key technologies for future large-scale location-aware services
covering a complex of multi-story buildings --- e.g., a big shopping mall and a
university campus --- is a scalable indoor localization technique. In this
paper, we report the current status of our investigation on the use of deep
neural networks (DNNs) for scalable building/floor classification and
floor-level position estimation based on Wi-Fi fingerprinting. Exploiting the
hierarchical nature of the building/floor estimation and floor-level
coordinates estimation of a location, we propose a new DNN architecture
consisting of a stacked autoencoder for the reduction of feature space
dimension and a feed-forward classifier for multi-label classification of
building/floor/location, on which the multi-building and multi-floor indoor
localization system based on Wi-Fi fingerprinting is built. Experimental
results for the performance of building/floor estimation and floor-level
coordinates estimation of a given location demonstrate the feasibility of the
proposed DNN-based indoor localization system, which can provide near
state-of-the-art performance using a single DNN, for the implementation with
lower complexity and energy consumption at mobile devices.Comment: 9 pages, 6 figure
A Fast Poisson Solver of Second-Order Accuracy for Isolated Systems in Three-Dimensional Cartesian and Cylindrical Coordinates
We present an accurate and efficient method to calculate the gravitational
potential of an isolated system in three-dimensional Cartesian and cylindrical
coordinates subject to vacuum (open) boundary conditions. Our method consists
of two parts: an interior solver and a boundary solver. The interior solver
adopts an eigenfunction expansion method together with a tridiagonal matrix
solver to solve the Poisson equation subject to the zero boundary condition.
The boundary solver employs James's method to calculate the boundary potential
due to the screening charges required to keep the zero boundary condition for
the interior solver. A full computation of gravitational potential requires
running the interior solver twice and the boundary solver once. We develop a
method to compute the discrete Green's function in cylindrical coordinates,
which is an integral part of the James algorithm to maintain second-order
accuracy. We implement our method in the {\tt Athena++} magnetohydrodynamics
code, and perform various tests to check that our solver is second-order
accurate and exhibits good parallel performance.Comment: 24 pages, 13 figures; accepted for publication in ApJ
A Syllable-based Technique for Word Embeddings of Korean Words
Word embedding has become a fundamental component to many NLP tasks such as
named entity recognition and machine translation. However, popular models that
learn such embeddings are unaware of the morphology of words, so it is not
directly applicable to highly agglutinative languages such as Korean. We
propose a syllable-based learning model for Korean using a convolutional neural
network, in which word representation is composed of trained syllable vectors.
Our model successfully produces morphologically meaningful representation of
Korean words compared to the original Skip-gram embeddings. The results also
show that it is quite robust to the Out-of-Vocabulary problem.Comment: 5 pages, 3 figures, 1 table. Accepted for EMNLP 2017 Workshop - The
1st Workshop on Subword and Character level models in NLP (SCLeM
Proto-Model of an Infrared Wide-Field Off-Axis Telescope
We develop a proto-model of an off-axis reflective telescope for infrared
wide-field observations based on the design of Schwarzschild-Chang type
telescope. With only two mirrors, this design achieves an entrance pupil
diameter of 50 mm and an effective focal length of 100 mm. We can apply this
design to a mid-infrared telescope with a field of view of 8 deg X 8 deg. In
spite of the substantial advantages of off-axis telescopes in the infrared
compared to refractive or on-axis reflective telescopes, it is known to be
difficult to align the mirrors in off-axis systems because of their asymmetric
structures. Off-axis mirrors of our telescope are manufactured at the Korea
Basic Science Institute (KBSI). We analyze the fabricated mirror surfaces by
fitting polynomial functions to the measured data. We accomplish alignment of
this two-mirror off-axis system using a ray tracing method. A simple imaging
test is performed to compare a pinhole image with a simulated prediction.Comment: 14 pages, 16 figure
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